The main two differences between any of the telecom standards are the frequency bands of operation and the modulation techniques. Some use frequency modulation, others use phase modulation, but all are some form of digital modulation. Just as there are disparate groups of auto enthusiasts that fervently believe their choice of car is the best, groups exist in the mobile communications cadres that think theirs is the best. Some systems are better than other form the standpoint of bandwidth efficiency, but that often comes with a degradation in call quality - particularly in weak signal conditions.

Take for example 16-QAM, where the receiver must correctly discern 16 unique phase angle/amplitude combinations to properly decode a data package. With a strong signal and a slow-moving platform (a walking talker), a phone can usually get near 100% of the data packages without the need for error correction or maybe have to request that the packet be re-sent from the tower. Same goes in the other direction from talker to tower.

Wne a user is moving quickly in a low power enviroment with lots of signal interferes around (electronics and physical), the system often breaks down. That is because of ambiguity in phase, amplitude. Take a simple BPSK signal in the same environment that only needs to discern between two separate states and it has a much better chance of surviving. Problem is, though, is that BPSK takes 8x the BW of a 16-PSK signal.

You can look up the technical difference between the various mobil phone systems, but in a nutshell this is the difference.